WO2017130552A1

WO2017130552A1 - Radiation image capturing device, control method for same, and program

Info

Publication number: WO2017130552A1
Application number: PCT/JP2016/084985
Authority: WO
Inventors: 晃介照井; 貴司岩下; 翔佐藤; 孔明石井
Original assignee: キヤノン株式会社
Priority date: 2016-01-27
Filing date: 2016-11-25
Publication date: 2017-08-03
Also published as: JP2017133929A; US20180317868A1; US10779777B2; JP6643909B2

Abstract

Provided is a radiation image capturing device including a sensor panel on which are arranged a plurality of pixels, each including a conversion element for detecting radiation, and a processing unit which generates an image corresponding to the number of radiation photons incident on each of the plurality of pixels, wherein, in an image capturing mode for generating an image formed by means of radiation that has been transmitted through a subject, the processing unit generates corrected signals by correcting the values of signals output from the conversion elements of each of the plurality of pixels, in accordance with a correction factor for converting the value of the signal output by the conversion element onto which a radiation photon has been incident, into a value corresponding to the energy value of said radiation photon, and generates an image on the basis of the number of corrected signals from pixels onto which radiation photons have been incident, from among the corrected signals from each of the plurality of pixels.

Description

Radiation imaging apparatus, control method thereof, and program

The present invention relates to a radiation imaging apparatus, a control method thereof, and a program.

2. Description of the Related Art A radiation imaging apparatus using a flat panel detector (FPD) made of a semiconductor material is known as an imaging apparatus used for medical image diagnosis by radiation and nondestructive inspection. Such a radiation imaging apparatus can be used as a digital imaging apparatus for capturing a still image, a moving image, or the like, for example, in medical image diagnosis.

Integral sensors that measure the total amount of charges generated by the incidence of radiation are widely known as radiation detection methods used in FPDs. As another type of sensor, there is a photon counting type sensor that measures the number of incident radiation photons. Patent Document 1 discloses a direct photon counting type sensor that directly detects radiation photons at each pixel using CdTe or the like. Patent Document 2 discloses an indirect photon counting type sensor that converts incident radiation photons into light by a scintillator and detects light converted from radiation at each pixel.

JP 2011-85479 A JP 2001-194460 A

In an FPD in which a plurality of pixels using a photon counting type sensor are arranged, if the sensitivity to incident radiation photons varies from pixel to pixel, the image quality of the captured radiation image deteriorates. In order to improve the image quality of the radiation image, it is necessary to correct variations in sensitivity to radiation photons for each pixel. Patent Document 1 discloses that sensitivity correction is performed using a count value of incident radiation photons. However, if the intensity of incident radiation photons is not uniform in the plane of the FPD, the accuracy of correction may be reduced. Further, Patent Document 2 does not disclose sensitivity correction.

An object of the present invention is to provide a technique for suppressing deterioration in image quality due to variation in sensitivity for each pixel in a radiation imaging apparatus using a photon counting type sensor.

In view of the above problems, a radiation imaging apparatus according to some embodiments of the present invention is incident on each of a plurality of pixels and a sensor panel including a plurality of pixels each including a conversion element for detecting radiation. And a processing unit that generates an image according to the number of the emitted radiation photons, wherein the processing unit receives radiation photons in an imaging mode that generates an image formed by radiation transmitted through the subject. Correction is performed by correcting the value of the signal output from each conversion element of the plurality of pixels according to the correction coefficient for converting the value of the signal output from the conversion element into a value corresponding to the energy value of the radiation photon. A signal is generated, and an image is generated based on the number of correction signals of pixels on which radiation photons are incident among correction signals of a plurality of pixels.

The above means provides a technique for suppressing a decrease in image quality due to variations in sensitivity for each pixel in a radiation imaging apparatus using a photon counting type sensor.

Other features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. In the accompanying drawings, the same or similar components are denoted by the same reference numerals.

The accompanying drawings are included in the specification, constitute a part thereof, show an embodiment of the present invention, and are used to explain the principle of the present invention together with the description.
The figure which shows the structural example of the radiation imaging device which concerns on this invention. The figure which shows the structure of the pixel of the radiation imaging device of FIG. The figure which shows the irradiation period and read-out period of the sensor panel of the radiation imaging device of FIG. , , The figure which shows the 1st process of the radiation imaging device of FIG. , The figure which shows the acquisition method of the captured image of the radiation imaging device of FIG. , , , The figure which shows the correction method of the captured image of the radiation imaging device of FIG. , , The figure which shows the acquisition method of the correction coefficient image of the radiation imaging device of FIG. The figure which shows the output of the signal of one pixel of the radiation imaging device of FIG. The figure which shows the imaging flow of the radiation imaging device of FIG. The figure which shows the modification of the imaging flow of FIG. The figure which shows the modification of the imaging flow of FIG. The figure which shows the modification of the imaging flow of FIG.

Hereinafter, specific embodiments of the radiation imaging apparatus according to the present invention will be described with reference to the accompanying drawings. The radiation in the present invention includes a beam having energy of the same degree or more, such as X-rays, β-rays, γ-rays, etc., which are beams formed by particles (including photons) emitted by radiation decay, such as X It can also include rays, particle rays, and cosmic rays.

<First Embodiment>
A radiation imaging apparatus according to some embodiments of the present invention will be described with reference to FIGS. FIG. 1 shows a configuration example of a radiation imaging apparatus 100 according to the first embodiment of the present invention. The radiation imaging apparatus 100 includes an imaging unit 104 that captures a radiation image and a processing unit 103. The radiation imaging apparatus 100 can constitute a radiation imaging system 110 together with a radiation source 101 that irradiates the radiation imaging apparatus 100 with radiation and an irradiation control unit 102 that controls the radiation source 101. Each of the irradiation control unit 102 and the processing unit 103 can be configured by a computer having a CPU, a memory, and the like. In the present embodiment, the irradiation control unit 102 and the processing unit 103 are configured separately, but are not limited thereto. For example, the irradiation control unit 102 may be integrated with the processing unit 103 and included in the radiation imaging apparatus 100. That is, the irradiation control unit 102 and the processing unit 103 may be configured by one computer having these functions.

The imaging unit 104 of the radiation imaging apparatus 100 includes a sensor panel 106 including a scintillator 105 that converts incident radiation into light and a plurality of pixels 120. The plurality of pixels 120 share the scintillator 105 with each other. Each of the pixels 120 includes a photodetector that detects light converted from radiation by the scintillator 105. That is, in this embodiment, in order to detect the incident radiation, the incident radiation is converted into light by the scintillator 105, and a signal corresponding to the intensity of the light is detected by a photodetector arranged in each of the pixels 120 as a conversion element. An indirect type conversion element that converts to is used. In the sensor panel 106, a plurality of pixels 120 are arranged in a two-dimensional array so as to form a plurality of rows and a plurality of columns. Each of the photodetectors of the pixel 120 outputs a signal (optical signal) having a value corresponding to the intensity of light converted by the scintillator 105 from radiation photons incident on the sensor panel 106 to the processing unit 103. Since the intensity of light converted by the scintillator 105 changes according to the energy of each radiation photon of the incident radiation, the value of the signal output from each photodetector of the pixel 120 is the incident radiation photon. It can be a signal value corresponding to the energy of. The radiation imaging apparatus 100 has a configuration for performing photon counting radiation imaging, and measures the number of incident radiation photons based on the light detection result.

The processing unit 103 exchanges signals and data with the imaging unit 104. Specifically, the processing unit 103 controls the imaging unit 104 to capture a radiographic image, and receives a signal obtained thereby from the imaging unit 104. receive. This signal includes a measurement value of radiation photons. For example, the processing unit 103 displays image data captured by radiation on a display unit (not shown) such as a display based on the measurement value. Is generated. At this time, the processing unit 103 may perform a predetermined correction process on the image data. The correction process will be described later. Further, the processing unit 103 can supply a signal for starting or ending radiation irradiation to the irradiation control unit 102.

FIG. 2 shows an equivalent circuit of the pixel 120 in the sensor panel 106 of the present embodiment. The pixel 120 can include a photoelectric conversion element 201 as a photodetector that detects light converted from radiation photons by the scintillator 105, and an output circuit unit 202. The photoelectric conversion element 201 can typically be a photodiode. The output circuit unit 202 can include an amplifier circuit unit 204, a clamp circuit unit 205, a sample hold circuit unit 207, and a selection circuit unit 208.

The photoelectric conversion element 201 includes a charge storage unit, and the charge storage unit is connected to the gate of the MOS transistor 204a of the amplifier circuit unit 204. The source of the MOS transistor 204a is connected to the current source 204c through the MOS transistor 204b. The MOS transistor 204a and the current source 204c constitute a source follower circuit. The MOS transistor 204b is an enable switch that is turned on when the enable signal EN supplied to the gate thereof becomes an active level to bring the source follower circuit into an operating state.

In the example shown in FIG. 2, the charge storage portion of the photoelectric conversion element 201 and the gate of the MOS transistor 204a constitute a common node, and this node is a charge that converts the charge stored in the charge storage portion into a voltage. Functions as a voltage converter. That is, the voltage V (= Q / C) determined by the charge Q stored in the charge storage unit and the capacitance value C of the charge voltage conversion unit appears in the charge voltage conversion unit. The charge-voltage converter is connected to the reset potential Vres via the reset switch 203. When the reset signal PRES becomes active level, the reset switch 203 is turned on, and the potential of the charge-voltage converter is reset to the reset potential Vres.

The clamp circuit unit 206 clamps the noise output from the amplifier circuit unit 204 by the clamp capacitor 206a according to the reset potential of the charge-voltage conversion unit. That is, the clamp circuit unit 206 is a circuit for canceling this noise from the signal output from the source follower circuit in accordance with the electric charge generated by the photoelectric conversion in the photoelectric conversion element 201. This noise may include kTC noise at reset. Clamping is performed by setting the clamp signal PCL to the active level to turn the MOS transistor 206b on, and then setting the clamp signal PCL to the inactive level to turn the MOS transistor 206b off. The output side of the clamp capacitor 206a is connected to the gate of the MOS transistor 206c. The source of the MOS transistor 206c is connected to the current source 206e via the MOS transistor 206d. The MOS transistor 206c and the current source 206e constitute a source follower circuit. The MOS transistor 206d is an enable switch that is turned on when the enable signal EN0 supplied to the gate thereof becomes an active level and puts the source follower circuit into an operating state.

A signal output from the clamp circuit unit 206 according to the electric charge generated by the photoelectric conversion in the photoelectric conversion element 201 is written as an optical signal into the capacitor 207Sb via the switch 207Sa when the optical signal sampling signal TS becomes an active level. It is. A signal output from the clamp circuit 206 when the MOS transistor 206b is turned on immediately after resetting the potential of the charge-voltage converter is a clamp voltage. This noise signal is written to the capacitor 207Nb via the switch 207Na when the noise sampling signal TN becomes an active level. This noise signal includes an offset component of the clamp circuit unit 206. The switch 207Sa and the capacitor 207Sb constitute a signal sample / hold circuit 207S, and the switch 207Na and the capacitor 207Nb constitute a noise sample / hold circuit 207N. The sample hold circuit unit 207 includes a signal sample hold circuit 207S and a noise sample hold circuit 207N.

When the drive circuit unit (not shown) drives the row selection signal VSR to the active level, the signal (optical signal) held in the capacitor 207Sb is output to the signal line 25S via the MOS transistor 208Sa and the row selection switch 208Sb. At the same time, a signal (noise) held in the capacitor 207Nb is output to the signal line 25N via the MOS transistor 208Na and the row selection switch 208Nb. The MOS transistor 208Sa constitutes a constant current source (not shown) provided in the signal line 25S and a source follower circuit. Similarly, the MOS transistor 208Na forms a source follower circuit and a constant current source (not shown) provided on the signal line 25N. The MOS transistor 208Sa and the row selection switch 208Sb constitute a signal selection circuit unit 208S, and the MOS transistor 208Na and the row selection switch 208Nb constitute a noise selection circuit unit 208N. The selection circuit unit 208 includes a signal selection circuit unit 208S and a noise selection circuit unit 208N.

The pixel 120 may include an addition switch 209 </ b> S that adds the optical signals of a plurality of adjacent pixels 120. In the addition mode, the addition mode signal ADD becomes an active level, and the addition switch 209S is turned on. Thereby, the capacitors 207Sb of adjacent pixels are connected to each other by the addition switch 209S, and the optical signals are averaged. Similarly, the pixel 120 may include an addition switch 209N that adds noises of a plurality of adjacent pixels 120. When the addition switch 209N is turned on, the capacitors 207Nb of adjacent pixels are connected to each other by the addition switch 209N, and noise is averaged. Adder 209 includes an addition switch 209S and an addition switch 209N.

The pixel 120 may have a sensitivity changing unit 205 for changing the sensitivity. The pixel 120 can include, for example, a first sensitivity conversion switch 205a and a second sensitivity conversion switch 205a 'and circuit elements associated therewith. When the first change signal WIDE becomes active level, the first sensitivity change switch 205a is turned on, and the capacitance value of the first additional capacitor 205b is added to the capacitance value of the charge-voltage converter. This reduces the sensitivity of the pixel 120. When the second change signal WIDE2 becomes an active level, the second sensitivity change switch 205a 'is turned on, and the capacitance value of the second additional capacitor 205b' is added to the capacitance value of the charge-voltage converter. This further reduces the sensitivity of the pixel 120. By adding a function for reducing the sensitivity of the pixel 120 in this way, it becomes possible to receive a larger amount of light and to expand the dynamic range. When the first change signal WIDE becomes active level, the enable signal ENw may be set to active level, and the MOS transistor 204a 'may be operated as a source follower instead of the MOS transistor 204a.

The optical signal output from the circuit included in the pixel 120 as described above may be supplied to the processing unit 103 after being converted into a digital value by an AD converter (not shown). The processing unit 103 processes this optical signal as a signal output from each pixel 120.

Next, driving of the radiation imaging apparatus 100 in the present embodiment will be described. FIG. 3 is a diagram illustrating the drive timing of the radiation imaging apparatus 100. The waveform in FIG. 3 represents a radiation irradiation period and a data DATA reading period with the horizontal axis as time. In FIG. 3, a radiation irradiation period is a period during which radiation is irradiated from the radiation source 101. During this period, the radiation incident on the sensor panel 106 is converted into light by the scintillator 105, and the photodetector of each pixel 120 outputs a signal corresponding to the intensity of the light. The readout period is a period in which the data DATA obtained during the radiation irradiation period is output from the sensor panel 106 to the processing unit 103. The radiation imaging apparatus 100 acquires a still image or a moving image by alternately performing a radiation irradiation period and a radiation reading period. In addition, the radiation imaging apparatus 100 according to the present embodiment acquires a still image or a moving image with a radiation irradiation period, a radiation reading period, a radiation non-irradiation period, and an offset reading period as one frame. The radiation imaging apparatus 100 can correct unnecessary offset information by subtracting the signal value acquired in the offset reading period from the signal value acquired in the radiation reading period.

Next, the detection of the pixel 120 at the position where the radiation photon is incident on the sensor panel 106 using FIGS. 4A to 4C and the photodetector of the pixel 120 according to the intensity of the light converted by the scintillator 105 from the incident radiation photon. A first processing method for correcting the value of the signal output from the above will be described. In the sensor panel 106 using an indirect type conversion element using the scintillator 105 to detect incident radiation photons, the light converted from the radiation photons by the scintillator 105 is diffused in the scintillator 105 and a plurality of pixels 120. Can be detected by a photodetector. For example, light generated by one radiation photon is detected across a plurality of pixels 120 as shown in FIG. 4A. At this time, an image generated by a signal output from the photodetector of each pixel 120 is an image as shown in FIG. 4B, for example. In this specification, as shown in FIG. 4B, an image obtained by light converted from one radiation photon by the scintillator 105 is referred to as a light emission image. The processing unit 103 analyzes the emission image to determine which radiation photon is incident on the scintillator 105 on which pixel 120, and the photodetector of the pixel 120 according to the position where the radiation photon is incident. The signal value output from can be corrected.

As a method of determining the position of a pixel on which radiation photons are incident, first, the processing unit 103 determines whether the value of a signal output from each pixel 120 is a signal having a value larger than a predetermined value. . This value can be a threshold value for determining whether or not the radiation photons are incident on the sensor panel 106 and the light converted by the scintillator 105 is detected by each pixel 120.

Next, the processing unit 103 identifies an aggregate of the pixels 120 that output a signal indicating that light converted from radiation photons is incident during the same period (frame) among the respective pixels 120. The aggregate includes a plurality of adjacent pixels 120 that output a signal indicating that light has been detected in the same period.

After identifying the aggregate, the processing unit 103 determines, based on the distribution of the aggregate, which of the pixels 120 included in the aggregate, the radiation photon is incident on the scintillator 105 on which pixel 120. In this embodiment, the photodetector of each pixel 120 outputs a signal having a value corresponding to the intensity of incident light. For this reason, for example, the processing unit 103 determines that the pixel 120 that outputs the highest signal value among the pixels 120 included in the aggregate is the pixel 120 at the position where the radiation photons are incident. Also good. Further, for example, the processing unit 103 may determine that the pixel 120 at the geometric gravity center position is the pixel 120 at the position where the radiation photons are incident in the arrangement of the aggregated pixels 120 on the sensor panel 106. In addition, for example, a distribution pattern of a light emission image when radiation photons are incident may be stored in the memory 130 of the processing unit 103 in advance. In this case, the processing unit 103 may determine the pixel 120 at the position where the radiation photons are incident by pattern matching with the light emission image of the aggregate.

After determining the pixel 120 at the position where the radiation photon is incident, the processing unit 103 corrects the value of the signal from the photodetector of each pixel 120 included in the aggregate according to the position where the radiation photon is incident. For example, the processing unit 103 performs correction to add the signal values output from the photodetectors of the pixels 120 included in the aggregate to the signal values of the pixels 120 at the positions where the radiation photons are incident. May be. For example, the processing unit 103 adds the signal values of only the pixel 120 determined to be at the position where the radiation photon is incident and the pixel 120 adjacent to the pixel 120 to the position where the radiation photon is incident. You may correct | amend as the value of the signal output from the certain pixel 120. FIG. In addition, the processing unit 103 corrects the signal from the photodetector of the pixels 120 other than the pixel 120 determined to be at the position where the radiation photons are incident in the aggregate to the same value as when light is not detected. May be. In addition, for example, when the processing unit 103 determines the pixel 120 at the position where the radiation photon is incident by performing pattern matching, the processing unit 103 further stores the corrected signal value in the light emission image stored in the memory 130. Alternatively, the signal value may be corrected based on this. As a result, as shown in FIG. 4C, an image is generated in which the pixel at the position where the radiation photon is incident is determined and the signal corresponding to the energy of the incident radiation photon is corrected. In this specification, the image in the stage of FIG. 4C is referred to as a radiation photon position determination image.

In addition, when no adjacent pixel 120 detects light with respect to one pixel 120 that has output a signal indicating that light has been detected, the one pixel 120 is located at a position where radiation photons are incident. Therefore, the necessity of correcting the signal from the pixel 120 can be reduced. For this reason, when no adjacent pixel 120 detects light for one pixel 120 that has output a signal indicating that the light detector has detected light, the processing unit 103 causes the one pixel 120 to be detected. However, the first process may not be performed.

In this embodiment, the sensor panel 106 using an indirect type conversion element (photodetector) using the scintillator 105 is used, but the radiation photon is directly converted into a signal corresponding to the energy of the incident radiation photon as the detection element. A direct conversion element for conversion may be used. When using a sensor panel using a direct conversion element, the processing unit 103 may determine that a pixel that outputs a signal having a value larger than a predetermined value is a pixel on which radiation photons are incident. Since the sensor panel using the direct conversion element does not use the scintillator 105, unlike the sensor panel using the indirect conversion element, light is unlikely to be detected by many pixels. For this reason, when the sensor panel using the direct type conversion element is used, the processing unit 103 may omit the first process.

Next, a method for generating a radiation image using the radiation photon position determination image is shown in FIGS. 5A and 5B. In an imaging mode for generating an image formed by radiation transmitted through the subject, the subject is irradiated with radiation from the radiation source 101. For example, imaging is performed over a plurality of frames while the subject is fixed, and the processing unit 103 reads out data DATA for each frame from the sensor panel 106. The read data DATA is subjected to the above first processing by the processing unit 103 to determine the position of the pixel 120 at the position where the radiation photons are incident, and to the signal output from the photodetector of the pixel 120. Correction is performed. By this processing, a plurality of radiation photon position determination images are obtained for each frame as shown in FIG. 5A. A radiation image shown in FIG. 5B can be generated from the position determination images of the plurality of radiation photons. As a method of generating a radiation image, the processing unit 103 counts the number of times a signal determined to have received a radiation photon is detected in each pixel 120, and the radiation shown in FIG. 5B based on the counted number. Generate an image. In the position determination image of the radiation photon for each frame in FIG. 5A, each bright spot indicates that a signal determined that the radiation photon has entered the pixel 120 is detected once. The number counted by each of the pixels 120 may be used as the pixel value of each pixel of the radiation image. Further, for example, the processing unit 103 counts the signals output when radiation photons are incident among the signals output from the photodetectors of the respective pixels 120 at a plurality of levels according to the values of the signals. May be. The processing unit 103 may generate a radiographic image based on the number of signals determined to be incident with radiation photons counted at the plurality of levels. In this case, the processing unit 103 may change the colors to be output at a plurality of levels, and synthesize a color radiation image having a color corresponding to the energy of each incident radiation photon. The radiation imaging apparatus 100 has energy resolution with respect to incident radiation photons by counting at a plurality of levels corresponding to the values of the signals.

In FIG. 5A and the drawings and explanations to be described later, for simplification of explanation, an image is shown for each frame. However, the processing unit 103 does not have to generate an image for each frame. A radiographic image is generated by counting the number of signals in which light converted from radiation is detected among signals output from the respective pixels 120.

Next, a radiographic image correction method according to this embodiment will be described with reference to FIGS. 6A to 6D. Each of the pixels 120 arranged on the sensor panel 106 does not always have a constant sensitivity to incident radiation energy. For example, due to variations in characteristics within the surface of the scintillator 105 and variations in gain with respect to the light of the photodetectors of the pixels 120, differences in sensitivity to the energy of incident radiation photons may occur in the respective pixels 120. Due to the difference in sensitivity, even when radiation photons having the same energy are incident on the sensor panel 106, a signal having a different value for each photodetector of the pixel 120 can be output. Therefore, the processing unit 103 follows the correction coefficient determined in the calibration mode to be described later for converting the value of the signal output from the photodetector of the pixel 120 into a value corresponding to the energy value of the incident radiation photon. to correct. The correction coefficient indicates the sensitivity of each pixel 120 to incident radiation photons. When the correction coefficients determined for the respective pixels 120 arranged on the sensor panel 106 are represented as an image, a correction coefficient image that is a collection of correction coefficients as shown in FIG. 6D is generated. In order to make the sensitivity uniform according to the correction coefficient (correction coefficient image), the processing unit 103 corrects the value of the signal output from the photodetector of each pixel 120.

6A to 6C show the order in which the processing unit 103 performs correction according to the correction coefficient on the image acquired for each frame to generate a radiation image. FIG. 6A shows the image of the light emission shown in FIG. 4B acquired for each frame. FIG. 6B shows a radiation photon position determination image generated by the first process from the light emission image of FIG. 6A. FIG. 6C shows a radiation image generated from a position determination image of a plurality of radiation photons acquired for each frame. Correction according to the correction coefficient may be performed at any stage of FIGS. 6A to 6C, but may be most effective when performed on the light emission image of FIG. 6A. The processing unit 103 corrects each signal value of the light emission image according to the correction coefficient, and generates a correction signal for each signal. By performing the first processing next using this correction signal, the determination of the position of the pixel 120 on which the radiation photons are incident and the correction of the value of the signal output from each photodetector of the pixel 120 are performed. Can be done more accurately. For this reason, the information regarding the energy of the radiation photon which entered into each pixel 120 of the radiation image generated from the position determination image of the radiation photon can be more accurate.

The above-described correction coefficient image acquisition method will be described with reference to FIGS. 7A to 7C. In the calibration mode in which radiation having a predetermined energy value is incident on the sensor panel 106 of the radiation imaging apparatus 100, the processing unit 103 determines a correction coefficient for each pixel 120 and acquires a correction coefficient image. The calibration mode is performed without placing a subject between the radiation source 101 and the sensor panel 106 so that radiation having a predetermined energy value enters the sensor panel.

First, the processing unit 103 acquires luminescence images in a plurality of frames while radiation having a predetermined energy value is incident. The processing unit 103 performs a first process from the plurality of light emission images shown in FIG. 7A and generates a radiation photon position determination image shown in FIG. 7B. Next, the processing unit 103 acquires, for each pixel 120, the value of the signal output from the photodetector of the pixel on which the radiation photon is incident from the position determination image of the radiation photon. Next, the processing unit 103 determines a correction coefficient based on the value of the signal from the pixel 120 on which the radiation photon is incident and the value corresponding to the case where the radiation photon having a predetermined energy value is incident. Specifically, the correction coefficient is determined so that the value of the signal from the pixel 120 on which the radiation photon is incident is converted into a value corresponding to the case where the radiation photon having a predetermined energy value is incident. The value corresponding to the incident radiation photon having a predetermined energy value may be a design value of a signal output from the photodetector of each pixel 120 with respect to the energy value of the incident radiation photon. Further, the value of the energy of the radiation photons incident on the sensor panel 106 can be acquired from, for example, the irradiation control unit 102 that controls the radiation source 101. A collection of correction coefficients determined by the processing unit 103 for each pixel 120 is a correction coefficient image shown in FIG. 7C. Moreover, since the energy of the incident radiation photon has a predetermined value, the first signal to be acquired may be only a value close to the design value. By acquiring a value close to the design value, it is possible to prevent the correction coefficient from being determined by noise that is not incident with radiation photons or irregular signals described later.

In order to determine the correction coefficient of each pixel 120, the processing unit 103 needs to acquire a signal output from the photodetector when radiation photons are incident at least once in each pixel 120. . In addition, the processing unit 103 acquires a plurality of signals output from the photodetector when radiation photons are incident on each pixel 120, and corrects each correction coefficient of the pixel 120 based on the statistics of the plurality of signals. May be determined.

Here, a method for determining a correction coefficient from a plurality of acquired signal statistics will be described. FIG. 8 shows a signal output for each frame when attention is paid to one pixel 120 in the radiation photon position determination image of FIG. 7B. The horizontal axis indicates the frame, and the vertical axis indicates the value of the signal output from the photodetector of the pixel 120. When the pixel 120 does not detect light converted from radiation photons, the value of the output signal is, for example, 0 (noise level). The processing unit 103 acquires a plurality of values of signals (first signals) when light converted from radiation photons is detected, and obtains statistics of these signals. As the statistic, for example, an average value, a median value, a mode value, or the like may be used. The obtained statistic indicates the energy sensitivity to the radiation photons of the photodetector of the pixel 120 of interest, and the processing unit 103 converts the obtained statistic value into a value corresponding to a predetermined energy value. To determine a correction factor. The processing unit 103 can acquire a correction coefficient image by determining a correction coefficient based on the statistic for each of the pixels 120 included in the sensor panel 106.

Since the amount of data for obtaining the statistic increases as the number of incident radiation photons on each pixel 120 increases, the processing unit 103 can acquire a more accurate correction coefficient image. For example, the processing unit 103 may determine a statistic from a signal output from the photodetector when 100 or more radiation photons are incident on each pixel 120.

Here, the number of radiation photons incident on each pixel 120 can be adjusted by appropriately setting the radiation dose from the radiation source 101 and the frame rate at which the sensor panel 106 captures an image. The radiation dose and frame rate may be set so as to avoid pileup. Pile-up means that a plurality of radiation photons are simultaneously detected by the photodetector of the same pixel 120 in the sensor panel 106, and the plurality of radiation photons are detected as a single radiation photon. In this case, the value of the signal output from the photodetector of the pixel 120 becomes an irregular value different from the value generated by one radiation photon, and an incorrect correction coefficient may be acquired. Also, when generating a radiographic image, the value of the signal output from the pixel 120 becomes an irregular value, and the image quality of the radiographic image may deteriorate. In order to avoid pile-up, the frame rate can be set so that the number of radiation photons incident on each pixel 120 is within one during the radiation exposure period of one frame. For example, pile-up can be suppressed by reducing the radiation dose and performing imaging with a high frame rate. For example, the operating frequency of the pixel 120 may be set within a range of 10 kHz to several MHz (for example, about 100 kHz). Further, for example, the radiation dose from the radiation source 101 may be set as a value when the tube voltage is about 100 kV and the tube current is about 10 mA. Here, for example, when the operating frequency of the pixel 120 is 100 kHz, when light generated by a plurality of radiation photons enters the same pixel 120 in a period of 0.01 milliseconds, the plurality of radiation photons are converted into a single radiation photon. A pile up to detect occurs.

Also, the statistics may be obtained using all signals output from the photodetector when radiation photons acquired in each of the pixels 120 are incident. Further, the statistic may be obtained by using a signal value for a part of the plurality of acquired signals. For example, the processing unit 103 determines a statistic by using a signal indicating a maximum / minimum value among a plurality of acquired signals or a remaining signal excluding several signals having higher / lower values. May be. Further, for example, when the distribution of a plurality of acquired signal values has a normal distribution, the processing unit 103 may determine a statistic using a signal having a value within a range of 3σ. As a result, for example, the value of an irregular signal that is generated by a direct hit that is directly detected by a photodetector without incident of cosmic rays or incident radiation photons being converted into light by the scintillator 105 affects the statistic. Can be suppressed. Similarly, an irregular signal value due to pile-up can be prevented from affecting the statistics.

In the present embodiment, radiation is performed using the same radiation source 101 in the calibration mode and the imaging mode. However, the calibration mode and the imaging mode may be performed without using the same radiation source. For example, in the calibration mode, the sensitivity to the energy of the radiation photons is acquired from the output value of the signal obtained by irradiating the monochromatic radiation and detecting the light converted from the radiation photons by the photo detector of the pixel 120. May be. In this case, since the energy of the radiation incident on each of the pixels 120 is constant, it is only necessary to acquire one signal due to the incidence of the radiation photons, and the statistics need not be obtained. By detecting one signal output by the incidence of radiation photons in the photodetector of each pixel 120, the processing unit 103 can determine an accurate correction coefficient and acquire a correction coefficient image. . Therefore, when determining the correction coefficient, it is not necessary to acquire a signal output from the photodetector when a plurality of radiation photons are incident on each pixel 120, and the calibration mode time can be greatly reduced. Thus, in the calibration mode for acquiring the correction coefficient image, for example, a monochromatic radiation source originating from a radioactive substance may be used, and in the imaging mode, the radiation source 101 that generates radiation by bremsstrahlung may be used.

Further, for example, the radiation of different energy may be irradiated, and the processing unit 103 may obtain the sensitivity to a plurality of radiation photons of different energy, and obtain a correction coefficient image therefrom. Further, the energy of the radiation photons when acquiring the correction coefficient image may be set in a range equal to or lower than the energy used when imaging the subject.

When the sensor panel 106 is irradiated with radiation from the radiation source 101, the intensity of radiation incident at the center and the end of the sensor panel 106, in other words, the number of incident radiation photons may be non-uniform. When the intensity of the radiation varies within the plane of the sensor panel, as shown in Patent Document 1, if the sensitivity is corrected using the number of radiation photons incident on each pixel, the radiation incident within the plane of the sensor panel. Since the number of photons varies, the accuracy of correction may decrease. However, in the present embodiment, the correction coefficient is determined directly from the signal value corresponding to the energy of the incident radiation photon output from each pixel 120. For this reason, even when the intensity of the incident radiation varies in the plane of the sensor panel 106, it is possible to determine a correction coefficient according to the sensitivity to the energy of the radiation photons of each pixel 120. As a result, it is possible to suppress degradation of the image quality of the radiation image due to variations in sensitivity for each pixel 120.

FIG. 9 shows a flow of radiographic image correction according to this embodiment. First, in the calibration process in the calibration mode, a correction coefficient is determined and an image is acquired. Specifically, the processing unit 103 performs a first process S1001 on a light emission image acquired without placing a subject between the radiation source 101 and the sensor panel 106, and a radiation photon position determination image. Is generated. Next, the processing unit 103 acquires a plurality of signals output when the radiation photons are incident on each pixel 120 using the position determination image of the radiation photons, and acquires statistics of the plurality of signals in step S1002. The processing unit 103 determines S1003 for each correction coefficient of the pixel 120 based on the acquired statistic. The correction coefficient (correction coefficient image) determined by the calibration process is stored in, for example, the memory 130 of the processing unit 103 and used for generating a correction signal when generating a radiation image. After acquiring the correction coefficient image, correction using the correction coefficient image is performed in the imaging process in the imaging mode. The imaging process can be said to be a normal imaging process in which a subject is placed between the radiation source 101 that irradiates the sensor panel 106 with radiation and the sensor panel 106. As described above with reference to FIG. 6A, the processing unit 103 acquires a plurality of light emission images for each frame from the imaging unit 104. Using the correction coefficient image for the light emission image, the processing unit 103 corrects the signal output from the photodetector of each pixel 120 and performs correction signal generation S1050. By the process of generating the correction signal, the in-plane characteristic variation of the scintillator 105 and the gain variation of the pixel 120 can be corrected. Thereafter, the processing unit 103 performs a first process S1001, acquires a plurality of radiation photon position determination images from the emission image correction signal, and combines the plurality of radiation photon position determination images S1052 to obtain a radiographic image. Is generated. A radiation image as shown in FIG. 6C is obtained by the step of generating the radiation image.

The correction coefficient may be determined by operating the calibration mode every time a radiographic image is taken. Further, for example, the determined correction coefficient may be stored in the memory 130 of the processing unit 103, and the correction coefficient may be read from the memory 130 and used when the processing unit 103 operates the imaging mode. By determining the correction coefficient for each radiographic image capture, appropriate correction can be performed for each radiographic image capture. Further, by storing the correction coefficient in the memory 130, it is not necessary to perform the calibration mode every time a radiographic image is captured, and the time required for imaging can be shortened.

In the present embodiment, the correction of the captured image is shown as an example used for the sensor panel 106 using an indirect type conversion element using the pixel 120 that detects light converted from radiation photons by the scintillator 105. It is not limited to this. The correction of the captured image of the present embodiment can also be applied to an imaging apparatus using a sensor panel using a direct conversion element that directly detects radiation photons at each pixel.

Second Embodiment
A radiation imaging apparatus according to the second embodiment of the present invention will be described with reference to FIGS. In the first embodiment, it is described that a correction coefficient for correcting variation in sensitivity with respect to radiation photons incident on each pixel 120 is determined, and correction according to the correction coefficient is performed when a radiation image is acquired in the imaging mode. did. In the present embodiment, correction for further suppressing deterioration of the image quality of a radiographic image in a radiation imaging apparatus using an indirect conversion element (photodetector) using a scintillator will be described. The radiation imaging apparatus 100 and the radiation imaging system 110 may be the same as those in the first embodiment described above.

FIG. 10 shows correction of a radiographic image that corrects a defective pixel generated by at least one of the scintillator 105 arranged in the sensor panel 106 and the photodetector included in each pixel 120 and suppresses deterioration of the image quality of the radiographic image. The flow of is shown. The processing unit 103 first acquires coordinate information representing the position of the defective pixel on the sensor panel 106. The coordinate information may be acquired when the processing unit 103 detects a defective pixel in the calibration mode. Further, the coordinate information of the defective pixel detected in advance may be stored in the memory 130 of the processing unit 103, and the processing unit 103 may acquire the coordinate information from the memory 130. Next, the processing unit 103 performs a second process S1004 that replaces the signal of the defective pixel based on the signal output from the photodetector of the pixel 120 adjacent to the defective pixel according to the coordinate information.

The defective pixel refers to a pixel 120 that has an abnormality in output due to, for example, an electrical failure such as a photodetector or a switch element that constitutes the pixel 120 or a scratch on the surface of the scintillator 105. For example, a pixel 120 that outputs a signal exceeding a predetermined threshold may be a defective pixel. Further, for example, a pixel 120 in which signal values output to adjacent pixels 120 are separated may be used as a defective pixel. Further, for example, a pixel 120 having a low linearity of a signal value output with respect to a change in incident light intensity may be used as a defective pixel. Further, for example, a pixel that does not always output a signal (a signal value is always a noise level) due to disconnection of a circuit constituting the pixel 120 may be a defective pixel.

Defective pixels may be detected by irradiating the sensor panel 106 with light and imaging before mounting the scintillator 105 on the sensor panel 106. In addition, before mounting the scintillator 105 on the sensor panel 106, defective pixels may be detected by performing imaging without irradiating the sensor panel 106 with light. In an image captured without irradiating light, a pixel that outputs a signal indicating that light has been detected may be a defective pixel. The coordinate information of the defective pixel detected before mounting the scintillator 105 is stored in the memory 130 of the processing unit 103.

Further, the defective pixel may be detected after the scintillator 105 is mounted on the sensor panel 106. In this case, in the calibration mode, the processing unit 103 may detect a defective pixel. The coordinate information of the detected defective pixel may be stored in the memory 130. For example, imaging may be performed with or without radiation, and the processing unit 103 may detect a defective pixel using a signal output from the photodetector of each pixel 120. Further, for example, the processing unit 103 may detect a defective pixel by causing the radiation imaging apparatus 100 to perform the same operation as that of the integral radiation imaging apparatus. That is, the processing unit 103 may detect a defective pixel from the total amount of charges generated by irradiating the radiation imaging apparatus 100 with radiation from the radiation source 101 and causing a plurality of radiation photons to enter the respective pixels 120. An advantage of detecting defective pixels using radiation after mounting the scintillator 105 on the sensor panel 106 is that defective pixels generated after mounting the scintillator 105 can be detected. The surface of the scintillator 105 is uneven, and defective pixels may occur when the sensor panel 106 on which the pixels 120 are formed and the scintillator 105 are bonded together. When defective pixels are detected using radiation, defective pixels can be detected more accurately than when defective pixels are detected using light before the scintillator 105 is mounted.

Further, the second process S1004 may be performed using coordinate information of defective pixels detected under a plurality of conditions. For example, before mounting the scintillator 105, both the coordinate information of the defective pixel detected by the imaging performed by irradiating light and the defective pixel detected by the imaging performed without irradiating the light and the coordinate information. One coordinate information may be combined with the coordinate information. Further, for example, the second process S1004 may be performed using both the coordinate information before mounting the scintillator 105 and the coordinate information after mounting the scintillator 105.

Using this coordinate information, the processing unit 103 performs a second process S1004 for replacing the signal of the defective pixel based on the signal from the photodetector of the pixel 120 adjacent to the defective pixel. For example, the signal of the defective pixel may be replaced with the signal value from the photodetector of the pixel 120 adjacent to the defective pixel, or the average of the signals from the photodetectors of the plurality of pixels 120 adjacent to the defective pixel. It may be replaced with a value. In the calibration mode, the processing unit 103 determines a correction coefficient S1003 using the signal of the defective pixel replaced by performing the second process S1004 on the light emission image. In the imaging mode, the processing unit 103 performs correction according to the correction coefficient on the signal of the defective pixel replaced by performing the second processing S1004 on the light emission image, and generates a correction signal S1050. Do. Thereafter, the processing unit 103 performs a first process S1001, acquires a plurality of radiation photon position determination images, and combines the plurality of radiation photon position determination images S1052 to generate a radiation image. By replacing the signal of the defective pixel before performing the first process S1001, the accuracy of the first process S1001 can be improved, and the deterioration of the image quality of the finally obtained radiographic image can be suppressed.

Next, FIG. 11 shows a radiographic image correction flow in which the processing unit 103 performs the third process S1005 in which the signal output from each photodetector of the pixel 120 is corrected according to the light sensitivity coefficient. Before mounting the scintillator 105 on the sensor panel 106, the sensor panel 106 is irradiated with light having a predetermined intensity. The photosensitivity coefficient is a coefficient for converting the signal value output from the photodetector of each pixel 120 into a value corresponding to the incident light of a predetermined intensity. That is, the photosensitivity coefficient represents the relationship between the intensity of light incident on the photodetector of the pixel 120 and the value of the optical signal output from the photodetector. The value corresponding to the case where light having a predetermined intensity is incident may be a design value of a signal output from the photodetector of each pixel 120 with respect to the intensity of the incident light. Based on the photosensitivity coefficient, the processing unit 103 corrects the variation in the gain of the output signal with respect to the intensity of the incident light of each pixel 120. The respective photosensitivity coefficients of the pixels 120 acquired before mounting the scintillator 105 on the sensor panel 106 are stored in the memory 130 of the processing unit 103, for example. In this embodiment, the photosensitivity coefficient, the above-described correction coefficient, coordinate information, and the like are stored in the same memory 130, but different memories may be prepared and stored. Further, in the first embodiment and the present embodiment, the memory 130 is arranged in the processing unit 103, but may be arranged outside the processing unit 103.

In the calibration mode, the processing unit 103 performs a third process S1005 using a light sensitivity coefficient on the light emission image, and corrects the gain of the signal from the photodetector of each pixel 120. After the third process S1005, correction coefficient determination S1003 is performed using the signal obtained by performing the first process S1001. In the imaging mode, the processing unit 103 performs correction according to the correction coefficient on the signal corrected by performing the third process S1005 on the light emission image, and generates a correction signal S1051. Since the gain variation of the pixel 120 is corrected by the third process S1005 using the photosensitivity coefficient, the generation S1051 of the correction signal can mainly correct the variation in the characteristics of the scintillator 105. Thereafter, the processing unit 103 performs a first process S1001, acquires a plurality of radiation photon position determination images, and combines the plurality of radiation photon position determination images S1052 to generate a radiation image. By performing the third process S1005 for correcting the gain of the pixel 120 before performing the first process S1001, the accuracy of the first process S1001 is improved, and the image quality of the radiation image finally obtained is reduced. Can be suppressed.

Also, as shown in FIG. 12, both the second process S1004 and the third process S1005 may be performed. The order of the second process S1004 and the third process S1005 may be performed first, but as shown in FIG. 12, it is more effective to perform the third process S1005 first. . This is because the second process S1004 for correcting the defective pixel uses the output of the pixel 120 adjacent to the defective pixel when the signal of the defective pixel is replaced. If the gain of the pixel 120 is corrected by the third process S1005 and the sensitivity of each pixel 120 to light is made uniform, the correction by the second process S1004 can be performed with higher accuracy.

<Other embodiments>
As described above, the correction coefficient determination and the correction using the correction coefficient may all be performed on the software of the processing unit 103. Further, for example, the configuration may be such that the circuit is provided outside the sensor panel 106 instead of software. In this case, for example, the circuit may be configured with an FPGA. In addition, among the processes performed by the processing unit 103, for example, at least a part of the process performed in the imaging mode may be performed by a circuit provided in each pixel 120 instead of software. In addition to the processing performed in the imaging mode, at least a part of the processing performed in the calibration mode may be performed by a circuit provided in the pixel 120. In the pixel 120, for example, a correction signal is generated according to a correction coefficient for a light emission image, and a signal in which light converted from radiation is detected is counted for each level of the value of the signal. A radiographic image may be generated based on the number of signals determined to have entered the counted radiation photons at each level. Further, the first process, the second process, and the third process may be performed on each pixel 120. In this case, each pixel 120 may be provided with a memory for storing correction coefficients, coordinate information, light sensitivity coefficients, and the like determined in the calibration mode.

Further, the present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus execute the program. It can also be realized by a process of reading and executing. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

The present invention is not limited to the above embodiment, and various changes and modifications can be made without departing from the spirit and scope of the present invention. Therefore, in order to make the scope of the present invention public, the following claims are attached.

This application claims priority on the basis of Japanese Patent Application No. 2016-013831 filed on Jan. 27, 2016, the entire contents of which are incorporated herein by reference.

Claims

Radiation imaging including a sensor panel in which a plurality of pixels each including a conversion element for detecting radiation are arranged, and a processing unit that generates an image according to the number of radiation photons incident on each of the plurality of pixels A device,
The processor is
In an imaging mode that generates an image formed by radiation transmitted through the subject,
In accordance with a correction coefficient for converting the value of the signal output from the conversion element into which the radiation photon has entered into a value corresponding to the energy value of the radiation photon, the signal output from the conversion element of each of the plurality of pixels Generate a correction signal by correcting the value,
A radiation imaging apparatus, wherein an image is generated based on the number of correction signals of pixels in which radiation photons are incident among the correction signals of the plurality of pixels.
The said processing part determines the pixel which output the signal in which the conversion element has a larger value than a predetermined value among these pixels as the pixel which the radiation photon entered. Radiation imaging device.
In the calibration mode in which a radiation photon having a predetermined energy value is incident on the radiation imaging apparatus, the processing unit
The correction coefficient is determined based on a value of a first signal from a conversion element on which radiation photons are incident among signals output from the conversion element and a value corresponding to the predetermined energy value. The radiation imaging apparatus according to claim 1 or 2.
The processing unit is in the calibration mode,
For each of the plurality of pixels,
Obtaining a plurality of the first signals;
The radiation imaging apparatus according to claim 3, wherein a correction coefficient is determined based on a plurality of statistics of the first signals.
5. The radiation imaging apparatus according to claim 4, wherein the processing unit uses any one of a median value, a mode value, and an average value of the plurality of first signals as the statistic.
6. The radiation imaging apparatus according to claim 4, wherein the processing unit determines the statistic from 100 or more first signals for each of the plurality of pixels.
The plurality of pixels share a scintillator that converts radiation into light,
Each of the plurality of pixels includes a photodetector for detecting the light,
The photodetector outputs an optical signal having a value corresponding to the intensity of the light incident on the photodetector;
The radiation imaging apparatus according to claim 3, wherein the optical signal is used as a signal output from the conversion element.
The processor is
Identifying an aggregate of pixels in which the light is incident during the same period among the plurality of pixels;
Determining which radiation photons are incident on the scintillator of which of the pixels included in the aggregate based on the distribution of the aggregate;
For each of the pixels included in the aggregate, further correcting a value of a signal output from the photodetector of the pixel according to a position where radiation photons are incident, and further performing a first process including:
8. The first signal is determined by performing the first process on a signal output from a photodetector of a pixel included in the aggregate in the calibration mode. The radiation imaging apparatus described.
The radiation imaging apparatus according to claim 8, wherein the processing unit performs the first processing on the correction signal in the imaging mode.
The processor is
Obtaining coordinate information of defective pixels caused by at least one of the scintillator and the photodetector;
In the calibration mode,
In accordance with the coordinate information, a second process of replacing the signal of the defective pixel based on a signal output from a photodetector of a pixel adjacent to the defective pixel is performed.
The radiation imaging apparatus according to claim 7, wherein the correction coefficient is determined using a signal of the defective pixel replaced by performing the second processing.
In each of the plurality of pixels, the processing unit outputs a signal output in a state where radiation is irradiated, a signal output in a state where radiation is not irradiated, and incidence of a plurality of radiation photons due to radiation irradiation. The radiation imaging apparatus according to claim 10, wherein the coordinate information is acquired by detecting the defective pixel based on at least one of a total amount of electric charges generated by.
The processor is
A first memory for storing the coordinate information;
The radiation imaging apparatus according to claim 10 or 11, wherein the second processing is performed according to the coordinate information stored in the first memory.
The said processing part performs correction | amendment according to the said correction coefficient with respect to the signal of the said defective pixel replaced by performing said 2nd process in the said imaging mode. The radiation imaging apparatus according to any one of the above.
The processor is
A photosensitivity coefficient for converting a value of a signal from a photodetector having light of a predetermined intensity incident on the photodetector among the plurality of pixels into a value corresponding to the light of the predetermined intensity is stored. With two memories,
In accordance with the photosensitivity coefficient stored in the second memory, a third process of correcting the value of the signal output from each photodetector of the plurality of pixels is performed,
14. The first signal is determined by performing the third process on a signal output from a photodetector of a pixel on which radiation photons are incident in the calibration mode. The radiation imaging apparatus according to any one of the above.
The radiation imaging apparatus according to claim 14, wherein the processing unit performs correction according to the correction coefficient on a signal corrected by performing the third processing in the imaging mode. .
The radiation imaging apparatus according to any one of claims 3 to 15, wherein, in the calibration mode and the imaging mode, radiation sources that irradiate the sensor panel with radiation are the same radiation source.
The processing unit, for each of the plurality of pixels in the imaging mode,
Among the correction signals, the correction signals of the pixels on which the radiation photons are incident are counted at a plurality of levels according to the values of the correction signals, respectively.
The radiation imaging apparatus according to claim 1, wherein a radiation image is generated based on the number of the correction signals respectively counted at the plurality of levels.
A method for controlling a radiation imaging apparatus including a sensor panel in which a plurality of pixels each including a conversion element for detecting radiation is arranged,
In order to form an image formed by radiation transmitted through the subject,
According to a correction coefficient for converting the value of the first signal output from the conversion element into which the radiation photon is incident into a value corresponding to the energy value of the radiation photon, the value is output from each conversion element of the plurality of pixels. Generating a correction signal by correcting the value of the signal;
Generating an image based on the number of the correction signals of pixels in which radiation photons are incident among the correction signals of the plurality of pixels;
A control method characterized by comprising:
A program that causes a computer to execute each step of the control method according to claim 18.